1. Field of the Invention
This invention relates to concentric pipe exothermic reactors in which exothermic reactions are carried out on solid catalysts.
2. Description of the Prior Art
In the production of ammonia, methanol and the like by vapor phase reactions performed on solid catalysts, various types of reactors have been heretofore proposed and reduced into practice in order to cool the catalyst layer or bed and properly control the distribution of temperature within the catalyst layer. Various methods have also been proposed in which not only the catalyst layer is merely cooled, but also heat of the reaction is transferred to cooling media and effectively utilized.
In continuous reactors for carrying out exothermic reactions, it is ordinary that in the vicinity of an outlet of the catalyst bed where the reaction proceeds, a concentration of reaction product becomes higher and the reaction comes closer to an equilibrium than in the vicinity of the inlet, with a lowering of the reaction rate. In the neighbourhood of the outlet of the catalyst bed, therefore, it is favorable to lower the reaction temperature, to permit the reaction system to be kept away from the equilibrium of the reaction, and to prevent the lowering of the reaction rate. At a portion near the inlet of the catalyst bed where a concentration of reaction product is sufficiently low, the system is known to be far from the equilibrium state of the reaction, so that higher temperatures are preferred.
Thus, when exothermic reactions are carried out, it is the most favorable to lower the temperature of the catalyst bed as the reaction proceeds.
FIG. 1 shows the relation between concentration of a reaction product (mol%) and reaction gas temperature, in which curve N indicates an equilibrium line for the reaction. The region at the right and upper side of the curve N is a region which cannot thermodynamically exist. Curve M is a line indicating a maximum reaction rate, showing the course of maximizing the reaction rate. In general, the maximum reaction rate line M is approximately parallel to the equilibrium line N. When the temperature distribution within the catalyst bed or the temperature distribution during the course of the reaction ideally moves on or along the curve M, the amount of catalyst required to obtain an intended reaction product becomes minimal.
In known tubular reactors such as described, for example, in Japanese Patent Publication No. 57-38568 which are cooled only by boiling water, the temperature of boiling water which is a cooling medium is approximately uniform throughout the reactor, with the problem that the temperature distribution of the catalyst bed becomes approximately uniform.
In order to improve the conditions of a reaction equilibrium, there has been proposed in Japanese Laid-open Patent Application No. 57-53420 a reactor in which in order to lower the temperature at a portion near the outlet of catalyst bed, supercooled water is fed to the portion near the outlet. In this case, the feed of supercooled water ensures the lowering of the temperature at or near the outlet of the catalyst bed. Because the transmission of heat from the supercooled water depends only on the heat transmission by convection, the cooling effect is much more reduced than the effect produced by the transmission of heat by boiling of water. This is industrially disadvantageous in leading to an increase of a required cooling area. If, on the contrary, the flow rate of cooling water is raised in order to suppress the reduction of the cooling effect, an increased pressure loss of the fluid is entailed, leading to the problem that a power requirement for pumping the water increases.
Because of the recent development of copper catalysts for the methanol synthetic reaction, catalysts which have very high activity even at low temperatures have been industrially produced. However, use of highly active catalysts results in a large quantity of heat of generation per unit amount of catalyst, requiring a larger cooling area and a more effective cooling technique. Especially, when copper catalysts used for the synthetic reaction of methanol are employed at temperatures over 300.degree. C. over a long period, their activity deteriorates, so that a sharp temperature rise within the catalyst bed is unfavorable not only from the standpoint of choice of material for the reactor, but also from the standpoint of deterioration of the catalysts. In known tubular reactors in which boiling liquid is used for cooling, e.g. reactors described in Japanese Patent Publication No. 57-38568, when the boiling liquid is passed countercurrently with a reaction gas passed through the catalyst bed, a proportion by volume of a gas phase in the boiling liquid becomes highest in the vicinity of the inlet of the catalyst bed where the highest cooling effect is required, making the worst efficiency for heat transmission, with an attendant problem that a very sharp temperature rise takes place in the vicinity of the inlet of the catalyst bed.
Starting gases being fed into the catalyst bed should be preheated to a temperature level sufficient to allow the reaction to proceed in the catalyst bed. For instance, with copper catalysts used for the synthetic reaction of methanol, the starting gases should be preheated to over 150.degree. C. In general, sensible heat of the hot gas from the outlet of the catalyst bed is used for preheating the starting feed gases. These gases are preheated in the heat exchanger located downstream of a reactor. The temperature of the gas at the outlet of a reactor, i.e. a gas at the outlet of the catalyst bed, is generally over 200.degree. C. when the reactor is for synthesis of methanol. The sensible heat of the reactor outlet gas having such a high temperature level may be used not only for the preheating of starting feed gases, but also for heat recovery such as preheating of boiler feed as described in Japanese Laid-open Patent Application No. 58-83642. In the method described in the above Japanese Laid-open Patent Application, heat recovery at such a high temperature level essentially requires a heat recovery apparatus arranged parallel to a preheater for the starting feed gas with limited ranges in amount of heat being recovered in the respective apparatus.